Conventionally, when threaded closures are to be secured onto containers on a sealing line, a closure is placed atop a container and is turned to mechanically engage its threading with that of the container. Turning is continued to draw the closure down on the container to a fully tightened secured position. However, if the closure threading is not properly aligned or "started" with respect to the threading of the container when rotation is commenced, the threads can strip or the closure can cock on the container so severely that the closure cannot be tightened without damaging it. The problem of misalignment arises because the threads are helical or tilted with respect to the vertical center axis of the container; when the lower ends of the closure thread or threads just engage the upper ends of the container threads the helical angulation initially tends to cock or tilt the closure so that the axis of the closure is misaligned or skewed with respect to the axis of the container. If the closure is turned while in such cocked position it might or might not align itself, depending upon its rotational position, the extent of misalignment, the clearance between the threads, and other factors; but progressive tightening can strip the threads or jam or even split the closure. In a closure applying line running at hundreds of containers per minute, misalignment can seriously slow the rate. Thus it is very important, as a practical matter, that closure misalignment be minimized and that every closure be started on its respective container with its central axis in line with that of the container.
Some prior art closures have been pushed axially part way onto the threads of containers before being turned; the closure elastically flexes radially so that its threads can slip past the upper part of the container threads. This reduces the amount of angular turning that is required to fully secure the closure. However, the closures were susceptible to being pressed on too far, which made removal difficult, especially if the closure was applied hot. Moreover, those closures had no tamper evidencing means.
The problems caused by closure misalignment become more serious if the closure has frangible (rupturable) tamper indicating means around it. Such tamper indicating means may for example be in the form of a band separably attached to the lower edge of the closure skirt, having retaining means that positively engages below a locking rib or bead on the container. The retaining means engages (as by snapping) below the locking rib when the closure is applied. One example of such tamper indicating means is shown in U.S. Patent application Ser. No. 401,966, previously referred to, to which reference may be had. When the closure is unscrewed to remove it from the container, the retaining means abuts the locking rib which then blocks further upward movement, thereby rupturing frangible bridges or perforations between the tamper indicating band and the closure skirt. The separation (which may be either partial or total) of the tamper indicating means from the upper part of the closure is visually apparent and provides an indication that tampering has occurred.
Because the tamper indicating means is frangibly attached to the rest of the closure, it can rupture prematurely if the closure is tightened on the container while in a misaligned position. If the closure is canted on the container, its effective cross section (in a plane perpendicular to the container axis) is elliptical rather than circular. As a result of this the retaining means is stressed nonuniformly when it engages the locking rib; the stress is greater along the wider axis of the ellipse (where the end curvature is tighter) and can cause the tamper indicating means to separate from the closure even though the closure may later align and the threads properly engage. Thus it is especially important that a threaded closure having tamper indicating means be properly aligned on the container when it is tightened to secure it in place.